The present invention relates to ultrasonic testing of structural materials particularly of workpieces made of metal for the purposes of detecting internal flaws, defects, inclusions or the like.
Sheet and plate stock, for example, is tested by launching ultrasonic test pulses perpendicularly to one surface and by searching for reflections of the ultrasonic signal, for example, by a defect. The sheet stock or plate may be so tested in its entirety and progressively over one surface thereby covering the entire interior of the stock. The return signals are usually processed by restricting the detection of echoes which occur within a certain period of time only because other echoes are deemed to have causes other than defects. Typically, for example, the so called rear wall echo will always occur as the result of the reflection o the ultrasonic pulse by the surface opposite the surface into which the test pulse was launched. In the case of a uniform wall thickness (i.e., in the case of sheet and plate stock of a uniform thickness) the time between the launching of the pulse into the workpiece and the occurrence of the rearwall echo is quite constant. Therefore, the period of time within which to look for echoes from defects in the interior of the workpiece is well defined. The situation is different, however, when the wall thickness varies intentionally or otherwise. In such a case, one has to consider the minimum wall thickness as the criteria defining rear wall echo timing and any echo that occurs after the minimum period has elapsed will be suppressed. This means that a portion of the sheet stock in locations where the wall thickness exceeds the minimum will not be tested. Alternatively, one can detect any echo whenever it occurs and evaluate the total transit time to thereby exclude detected rearwall echoes from echoes resulting from defects. The problem could be solved, for example, through follow-up systems in which in very small steps and on a progressive basis one keeps track of the rearwall echoes to thereby delineate the wall thickness, and echoes occurring within the variable period delineating the wall thickness can then be interpreted as defects. However, tracking the actually occuring wall thickness just for that purpose is complicated and expensive. Moreover, this matter requires a high resolution pattern for testing the workpiece as a whole without actual improvement in test sensitivity. One has also tried to include so called jump detectors in which echoes of sequential test cycles are compared under the assumption that the wall thickness varies gradually, and in the case of a jump in transit time of echoes that jump is interpreted as a defect. However, this is not a reliable method and in fact nonexisting "defects" may be indicated.